【JAVA之WebSocket介绍】

一、WebSocket介绍

WebSocket协议是基于TCP的一种新的网络协议。它实现了浏览器与服务器全双工(full-duplex)通信——允许服务器主动发送信息给客户端。

 

The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code. The security model used for this is the origin-based security model commonly used by web browsers. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications that need two-way communication with servers that does not rely on opening multiple HTTP connections (e.g., using XMLHttpRequest or <iframe>s and long polling)

WebSocket协议支持（在受控环境中运行不受信任的代码的）客户端与（选择加入该代码的通信的）远程主机之间进行全双工通信。用于此的安全模型是Web浏览器常用的基于原始的安全模式。 协议包括一个开放的握手以及随后的TCP层上的消息帧。 该技术的目标是为基于浏览器的、需要和服务器进行双向通信的（服务器不能依赖于打开多个HTTP连接（例如，使用XMLHttpRequest或<iframe>和长轮询））应用程序提供一种通信机制。

 

二、WebSocket产生背景

简单的说，WebSocket协议之前，双工通信是通过多个http链接来实现，这导致了效率低下。WebSocket解决了这个问题。下面是标准RFC6455中的产生背景概述。

Historically, creating web applications that need bidirectional communication between a client and a server (e.g., instant messaging and gaming applications) has required an abuse of HTTP to poll the server for updates while sending upstream notifications as distinct HTTP calls.

This results in a variety of problems:

o The server is forced to use a number of different underlying TCP connections for each client: one for sending information to the client and a new one for each incoming message.

o The wire protocol has a high overhead, with each client-to-server message having an HTTP header. o The client-side script is forced to maintain a mapping from the outgoing connections to the incoming connection to track replies.

A simpler solution would be to use a single TCP connection for traffic in both directions. This is what the WebSocket Protocol provides. Combined with the WebSocket API , it provides an alternative to HTTP polling for two-way communication from a web page to a remote server.

The WebSocket Protocol is designed to supersede existing bidirectional communication technologies that use HTTP as a transport layer. Such technologies were implemented as trade-offs between efficiency and reliability because HTTP was not initially meant to be used for bidirectional communication (see [RFC6202] for further discussion).[1] 

长久以来, 创建实现客户端和用户端之间双工通讯的web app都会造成HTTP轮询的滥用: 客户端向主机不断发送不同的HTTP呼叫来进行询问。

这会导致一系列的问题：

1.服务器被迫为每个客户端使用许多不同的底层TCP连接：一个用于向客户端发送信息，其它用于接收每个传入消息。

2.有线协议有很高的开销，每一个客户端和服务器之间都有HTTP头。

3.客户端脚本被迫维护从传出连接到传入连接的映射来追踪回复。

一个更简单的解决方案是使用单个TCP连接双向通信。 这就是WebSocket协议所提供的功能。 结合WebSocket API ，WebSocket协议提供了一个用来替代HTTP轮询实现网页到远程主机的双向通信的方法。

WebSocket协议被设计来取代用HTTP作为传输层的双向通讯技术,这些技术只能牺牲效率和可依赖性其中一方来提高另一方，因为HTTP最初的目的不是为了双向通讯。（获得更多关于此的讨论可查阅RFC6202)

 

三、WebSocket产生背景实现原理

在实现websocket连线过程中，需要通过浏览器发出websocket连线请求，然后服务器发出回应，这个过程通常称为“握手” 。在 WebSocket API，浏览器和服务器只需要做一个握手的动作，然后，浏览器和服务器之间就形成了一条快速通道。两者之间就直接可以数据互相传送。在此WebSocket 协议中，为我们实现即时服务带来了两大好处：

1. Header

互相沟通的Header是很小的-大概只有 2 Bytes

2. Server Push

服务器的推送，服务器不再被动的接收到浏览器的请求之后才返回数据，而是在有新数据时就主动推送给浏览器。

 

 

四、WebSockets 握手

客户端和服务器端TCP连接建立在HTTP协议握手发生之后。通过HTTP流量调试，很容易观察到握手。客户端一创建一个 WebSocket实例，就会出现如下请求和服务器端响应： 

注意: 我们只录入了WebSocket握手所用到的HTTP头。

 

请求:

GET /byteslounge/websocket HTTP/1.1 

Connection: Upgrade 

Upgrade: websocket 

Sec-WebSocket-Key: wVlUJ/tu9g6EBZEh51iDvQ==

 

响应:

HTTP/1.1 101 Web Socket Protocol Handshake 

Upgrade: websocket 

Sec-WebSocket-Accept: 2TNh+0h5gTX019lci6mnvS66PSY=

注意：进行连接需要将通过Upgrade and Upgrade将协议升级到支持websocket HTTP头的Websocket协议。服务器响应表明请求被接受，协议将转换到WebSocket协议（HTTP状态码101）:

HTTP/1.1 101 Web Socket Protocol Handshake